The signal surely is there. It's present since Qt 3.0 so it's not the matter of version... You can take a look at the docs: QButton::stateChanged(int). Try some other way of connecting the signal or attach the ui file here and I will make that connection for you.

EDIT: What do you mean "action ZoomIn"? You said you were connecting a button, not an action... I am talking about connecting the button's stateChanged signal to some signal/slot (whatever it is, be it even the action's activated() signal).